The present invention relates to a vertical type screening machine for granular material such as, for example, cereal grain (rice, wheat, soybean or the like), granular ore, granular industrial products, and the like.
Japanese Patent Laid-Open No. 57-159583 laid open to public inspection on Oct. 1, 1982 discloses a vertical type screening machine for granular material, which includes a housing and a rotary drum assembly rotatably mounted within the housing. The rotary drum assembly includes a drum having a vertical rotary axis and a helical blade helically wound around a circumferential surface of the drum and secured thereto. A cylindrical screen member is disposed between the housing and the drum assembly in concentric relation to the drum. The screen member cooperates with a peripheral wall of the housing to define therebetween a first chamber and cooperates with the circumferential surface of the drum to define therebetween a second chamber. A supply hopper comunicates with a bottom of the second chamber, allowing granular material to be screened, to be supplied into the second chamber. The helical blade is wound around the circumferential surface of the drum in such a direction as to allow the granular material fed to the bottom of the second chamber to be moved toward a top of the second chamber against gravity when the rotary drum assembly is drivingly rotated. When the granular material is moved from the bottom to the top of the second chamber by the rotation of the rotary drum assembly, the graunular material is moved radially outwardly toward the screen member by the rotary drum assembly, and granular material having a relatively small size contained in the supplied granular material is introduced into the first chamber through the screen member. The granular material of relatively small size within the first chamber is discharged from the bottom of the first chamber. The remaining graunular material within the second chamber is discharged from the top of the second chamber.
As described above, in the conventional vertical type screening machine, the granular material supplied into the bottom of the second chamber is moved from the bottom to the top of the second chamber against gravity by the helical blade secured to the drum upon the rotation thereof. This causes such a problem that the movement of the graunular material against gravity inevitably increases a power required to drivingly rotate the rotary drum assembly, to thereby decrease working efficiency of the machine and such a problem the the granular material would be damaged because the granular material urged against the screen member is forcibly pushed upwardly by the helical blade.
In addition, in the above-described vertical type screening machine of the prior art, as an amount of the granular material supplied into the bottom of the second chamber is decreased, a thickness of the granular material layer on an upper surface of the helical blade is decreased, and a pressure of the granular material layer acting on the upper surface of the helical blade is decreased, so that a centrifugal force acting on the granular material due to the frictional contact force of the granular material against the upper surface of the helical blade is decreased to reduce screening efficiency and screening accuracy.